Summary of Significant Accounting Policies (Changes in AOCI Balances by Component) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Balance	$ 1,420,227	$ 1,365,505	$ 1,247,535
Other comprehensive income (loss)	(3,248)	(547)	(8,239)
Balance	1,553,023	1,420,227	1,365,505
Accumulated Other Comprehensive (Loss) Income [Member]
Balance	(42,165)	(41,618)	(33,379)
Other comprehensive income (loss)	(3,248)	(547)	(8,239)
Other comprehensive loss before reclassifications	(4,347)
Amounts reclassified from AOCI	1,099
Balance	(45,413)	(42,165)	(41,618)
Defined Benefit Pension Plan [Member]
Balance	(38,242)	(40,268)	(32,113)
Other comprehensive income (loss)		2,026	(8,155)
Other comprehensive loss before reclassifications	(3,722)
Amounts reclassified from AOCI	1,099
Balance	(40,865)	(38,242)	(40,268)
Foreign Currency Translation [Member]
Balance	(4,603)	(1,389)	(751)
Other comprehensive income (loss)		(3,214)	(638)
Other comprehensive loss before reclassifications	(261)
Balance	(4,864)	(4,603)	(1,389)
Unrealized Gain (Loss) In Fair Value Of Investments [Member]
Balance	656	(110)	(315)
Other comprehensive income (loss)		766	205
Other comprehensive loss before reclassifications	(340)
Balance	316	656	(110)
Unrealized Gain (Loss) In Fair Value Of Interest Rate Swap [Member]
Balance	24	149	(200)
Other comprehensive income (loss)		(125)	349
Other comprehensive loss before reclassifications	$ (24)
Balance		$ 24	$ 149